For many audio applications, the ideal sound radiator may be characterized as a dimensionless full bandwidth omni-directional pulsating sphere also referred to as a ‘point source’. However, it is in practice impossible to provide such sound radiation characteristics and attempts to approach such an ideal sound generation has proved difficult and challenging as the requirements tend to be conflicting. For example, it is difficult for a very small speaker (i.e. approaching a dimensionless speaker) to move large amounts of air which is required for reproduction of bass frequencies at significant sound levels.
Traditional loudspeaker boxes typically contain two or more transducers that are aligned vertically and which partly share the reproduction of the same frequency range around the cross-over region. This tends to result in highly directional speakers which exhibit strong interference patterns in the vertical plane.
An example of a loudspeaker design is described in Patent Cooperation Treaty patent publication WO2006/097857. The disclosed loudspeaker design uses a low frequency loudspeaker combined with a high frequency loudspeaker which is mounted with a high distance to the low frequency speaker. Specifically, the two loudspeakers must be arranged with a distance of at least twice the wavelength of the cross-over frequency between the loudspeakers.
The system of WO2006/097857 has a number of interesting characteristics. For example, the arrangement tends to have low directivity at frequencies that are reproduced independently by the low frequency loudspeaker and the high frequency loudspeaker. Also, the interference between the loudspeakers tends to be perceived at a low level. The system tends to provide a reproduction of a sound stage that is very wide and deep and in which the speaker boxes are hardly perceived as being present.
Thus, the system of WO2006/097857 tends to provide a very immersive listening experience with the speakers seeming to blend in the soundstage. However, a disadvantage of the design is that it results in large speakers that are typically only suitable for use as large floor standing speakers.
Another example of a speaker design is the use of co-axial speaker arrangements wherein a high frequency transducer is placed in front of, or in the middle of, a low frequency transducer. The transducers are typically arranged to point directly towards the desired listening position with a coincidence of the perceived acoustic centres of both transducers. However, such speakers tend to be affected by reflection of the high frequency wave on the low frequency transducer surface resulting in a high directional directivity pattern and are therefore unsuitable for applications seeking to generate a point source audio radiation.
A modified co-axial speaker arrangement is provided in United States Patent application publication US 2003/0179899A1 which discloses a coaxial arrangement of a high frequency tweeter and a wide bandwidth loudspeaker. The coaxial arrangement is arranged in a partially upfiring configuration and reflectors are provided that reflect the upwards angled sound in a horizontal direction thereby providing a reduced directivity. However, although the speaker design may provide suitable characteristics in many embodiments, it tends to have some associated disadvantages. For example, the design is complex and sensitive to variations in the specific dimensions. For example, the reflectors must be carefully designed, manufactured and mounted to provide the desired effect. Accordingly, manufacturing tends to be suboptimal and/or costly. Also, the speaker arrangement tends to not provide optimal sound quality in some applications. Specifically, the reliance on reflected sound tends to result in a less focussed sound image being provided to the listener.
Another example of large floor standing speakers is the “Pluto” speaker designed by Linkwitz Labs.
The Pluto loudspeaker uses a frontfiring wide bandwidth sound transducer together with a low frequency woofer. The woofer assists the wide bandwidth sound transducer at low frequencies. The cut-off frequency between the wide bandwidth sound transducer and the low frequency woofer is at 1 kHz. In the design, the wide bandwidth sound transducer is implemented by a relatively large loudspeaker supported by a high acoustic load provided by a relatively large tube in which the wide bandwidth sound transducer is mounted. Indeed, in order to provide the desired audio characteristics, the speaker design requires a relatively large frontfiring sound transducer which is coupled with a substantial acoustic load thus requiring the frontfiring sound transducer to be mounted in a relatively large enclosure. In the design, the low frequency woofer is arranged in an upfiring configuration.
The Pluto loudspeaker may provide suitable performance for many audio applications but the reproduced sound quality tends to be suboptimal and a relatively high directivity results from the design. Notably, some reflections and diffraction from the woofer sound wave will occur onto the top part, which has non negligible dimensions compared to the wavelength around crossover frequency. The use of a large drive unit for the high-frequency also results in higher directivity. Also, the design is a large and floor standing loudspeaker which is unsuitable for many applications.
Hence, an improved speaker arrangement would be advantageous and in particular a speaker arrangement allowing reduced speaker size, reduced cost, facilitated manufacturing, increased design flexibility, improved audio quality, facilitated deployment, increased point source approximation and/or improved performance would be advantageous.